Small wars, or guerrilla wars, had an enormous impact in the age of Napoleon. Fought by peasants with access to land and resources, guerrilla wars in Haiti and Spain, in particular, reshaped the world in ways as profound as any of the major regular campaigns. They bled and demoralized the French and set the stage for the emergence of new nations in the Americas. This essay examines the two successful guerrilla wars in Haiti and Spain and compares them to two failed guerrilla wars in Calabria and the Tyrol in order to identify the key factors determining success or failure by guerrilla forces. Among the keys to success were: the geo-strategic importance of the theater of war; mobilizing ideologies; the presence of imperial troops for a long period of time with all of the resulting violence that implies; the reliance of imperial troops on requisitions in the countryside; the presence of strong allies; the impact of disease; and, above all, the presence of socio-economic conditions that both motivated peasants to take up arms to defend their families, land, and resources against long odds and that supplied peasants with the wherewithal to survive the French counterinsurgency.